PROJECT SUMMARY Aortic stenosis (AS) is a degenerative heart condition characterized by fibrosis and narrowing of the aortic valve. AS causes increased wall shear stress across the aortic valve, making the heart work harder to pump blood through the narrowed valve opening. These effects can cause heart failure and death. The only current treatment for AS is valve replacement, an invasive and risky procedure. Our long-term goal is to improve the understanding and treatment of fibrosis, which underlies AS. Transforming growth factor-?1 (TGF-?1) is a multifunctional cytokine that induces pathologic organ fibrosis in many disorders. Cells secrete latent (L)TGF-?1, and increased plasma levels of LTGF-?1 have been associated with AS in humans and in a mouse model of AS. We showed that wall shear stress can dramatically activate LTGF-?1 released from platelets in vitro. However, the mechanism of TGF-?1 activation in vivo and its signaling on valvular cells in AS remain poorly understood. The objective of this application is to determine the mechanism of LTGF-?1 activation in vivo and identify the cell types to which TGF- ?1 signals, leading to their mesenchymal transition and subsequent AS progression. Platelets are the major source of plasma LTGF-?1, as they contain more than 100 times as much LTGF-?1 as other cell types, and our preliminary data show that platelets are physically attached to isolectin B4-positive valvular cells, presumably valvular endothelial cells (VEC) undergoing mesenchymal transition and giving rise to collagen-producing myofibroblasts in an AS mouse model. These results led to our central hypothesis that wall shear stress activates platelet-derived TGF-?1, which stimulates isolectin B4-positive cells (VECs and/or macrophages) to undergo mesenchymal transition, thereby producing excessive collagen and leading to AS progression. Furthermore, TGF-?1 may represent a previously unrealized clinical indicator of AS progression. We propose the following Specific Aims: (1) Determine whether wall shear stress activates LTGF-?1 in vivo; and (2) Determine whether TGF-?1 signaling in valvular cells leads to mesenchymal transition and AS progression. Our proposed studies will clarify the mechanism of TGF-?1 activation in vivo and the cell types on which TGF-? signals, leading to mesenchymal transition and AS progression. We will use innovative, sensitive techniques to evaluate AS development in mouse models and to measure active TGF-?1 levels in plasma samples from AS patients. Furthermore, the proposed research will explore the mitigation of AS progression using two low-dose small-molecule inhibitors of TGF-?1 activation and signaling. Our pre-clinical assessment of TGF-?1 activation and signaling in AS patient samples will be crucial for prospective clinical studies on the use of TGF-?1 as a biomarker of AS. Overall, our proposed studies will provide a more complete understanding of the role of TGF-?1 in AS, which may establish TGF-?1 as a novel target for drug development and a potential biomarker of AS and other fibrotic diseases.